1. Field of the Invention
The present invention relates to an ultrasonic motor which includes a vibrator and a moving member in pressure contact with the vibrator, in which circular vibrations are formed in the vibrator by combining at least two vibrations excited therein using an electro-mechanical energy converter, such as a piezoelectric element, and in which the moving member is moved relative to the vibrator to obtain mechanical energy.
2. Description of the Related Art
The applicant of the present invention has proposed in Japanese Patent Application No. 2-206241 an ultrasonic motor shown in FIG. 11. In FIG. 11, reference numeral 11 denotes a vibrator made of a metallic material, and comprising a plurality of piezoelectric elements 3 which are interposed between cylindrical metallic blocks and which are held and fixed therebetween by fastening the metallic blocks using a bolt 7. In the vibrator 1, two bending vibrations (bending modes) intersecting at right angles are excited by applying AC voltages having different phases to the drive piezoelectric elements of the piezoelectric elements, and are combined to form a circular or elliptical rotational driving vibration at the tip of the vibrator 1. Namely, the application of AC signals produces a plurality of vibrations having a phase difference in time therebetween in a plurality of planes in the vibrator 1. The vibrator 1 has a peripheral groove formed in the upper portion thereof so as to increase the displacement of the drive vibration.
Reference numeral 2 denotes a rotor brought into pressure contact with the vibrator 1 by a pressure spring S to obtain a frictional driving force. Reference numeral 6 denotes a bearing for rotatably supporting the rotor 2.
The vibrator of the ultrasonic motor configured as described above is made of a material, e.g., free-cutting brass of JIS C3601 consisting of 59 to 63% copper, 1.8 to 3.7% lead, and the balance Zn, which can easily be mechanically processed and which preferably exhibit low periodic damping.
However, in the ultrasonic motor having the above structure, in order to form an ideal circular movement at the axis of the vibrator by combining the two excited vibration modes (sine mode and cosine mode) and transmit a constant frictional driving force to the rotor in pressure contact therewith, it is necessary to match the resonance frequencies of the two vibration modes. Otherwise, nonuniformity occurs in the locus of the elliptical vibration formed on the peripheral drive surface of the vibrator 1 by the driving vibration on the drive surface thereof, as viewed from the side of the vibrator 1. Thus, a constant frictional driving force cannot be transmitted to the rotor, thereby decreasing the efficiency of the motor.
Although attempts have been made to match the resonance frequencies of the two vibration modes in the vibrator, it is difficult to match the resonance frequencies of both vibration modes.
As a result of an examination of the cause of this mismatch between the resonance frequencies, the inventors have found that the mismatch between the resonance frequencies is caused by a mismatch between the natural frequencies of the respective vibration modes in the vibrator.
In conventional systems, if the natural frequencies of the respective vibrations in the two vibration systems intersecting at right angles are different, then, a notch may be formed in a portion of the vibrator, or a mass may be added thereto in order to match the natural frequencies. The applicants of this invention previously have filed an application in which such an external load is provided on a vibrator.
However, it is a drawback to provide such an external load because of differences between the structural and operating conditions of various vibrators. It is thus desirable to match the natural frequencies of the two vibrations intersecting at right angles by appropriately selecting a material for the vibrator.
The inventors thus performed research on the metallic material of a vibrator with respect to not only the composition of the metallic material but also the microstructure thereof. As a result, the inventors found that the mismatch between the natural frequencies is caused by the anisotropy of the crystal structure, particularly the anisotropy of the Young's modulus, which is possessed by the metallic material of the vibrator.
Another type of ultrasonic motor is disclosed in Japanese Utility Model Laid-Open No. 63-202188.
In this ultrasonic motor, a vibration in a longitudinal mode and a vibration in a torsional mode are axially excited in a vibrator, and are combined to form a circular vibration in the vibrator. This type of vibrator is also required to have a match between the resonance frequencies of the respective vibration modes.
However, this ultrasonic motor has the same drawback as that of the vibrator of the type in which bending vibrations are combined.